Discussion
Our findings illustrated differences in survival of different types of epicardial leads; bipolar steroid-eluting leads had considerably better survival rate than unipolar non-steroid-eluting ones, by more than 85% lower risk of failure in 48 months. However, no other lead and patient characteristics were revealed to be associated with lead failure. In this order, epicardial leads function and survival was similar between adults and children.
Epicardial lead implantation is preferable in small children, patients with congenital heart disease with right-to-left shunts, intention to preserve the venous access mainly in the ones with difficult accessibility, and prevention of venous thrombosis. However, recent advances in epicardial leads and developed bipolar steroid-eluting ones have demonstrated improved early pacing and sensing thresholds, as well as the long-term function of the lead, which is comparable with endocardial ones.1,2,8
We illustrated that bipolar steroid-eluting epicardial leads remained functional significantly better than unipolar non-steroid-eluting ones. Some studies investigated bipolar and unipolar leads pacing characteristics in the short-term, implying that the bipolar group had fewer sensing and pacing failure.10,11 The probable underlying mechanism has been declared to be related to the coaxial design of Medtronic bipolar leads, which result in a lower chance of stray signal pick up and better conduction performance.10 Furthermore, the pacing threshold illustrated to be more stable regarding bipolar steroid-eluting leads in all chambers in the long-term.2 Our finding also demonstrated that bipolar leads have better long-term pacing characteristics because of the significant rise observed in pacing threshold of unipolar leads in the right atrium and left ventricle but not regarding bipolar group and significantly greater pacing threshold of left ventricular unipolar leads at follow up. Meanwhile, primary pacing characteristics or energy threshold (ET) which was a predictive factor of lead failure in a study did not affected the lead survival in the current study.1 On the other hand, the observed difference between bipolar steroid-eluting and unipolar non-steroid-eluting might be related to the variation of the steroid elution. In this order, steroid-eluting leads were declared to have better long-term functions.1,12
A closer look at the mechanism of steroid impact reveals that addition of dexamethasone (steroid) to a lead can decrease inflammation of the electrode-tissue interface, which is the responsible factor for the rise in the acute pacing threshold. Furthermore, in the long-term, the inflammation might result in the formation of a scar or fibrous capsule in the electrode-tissue interface, which causes a higher pacing threshold.13 Even though, it is noteworthy that steroid elution leads have been shown not to be an influential factor in a previous study in particular when comparing epicardial and endocardial leads.14 It could be concluded that steroid elution and being bipolar could be both associated with better long-term function; however, more extensive studies that compare steroid elution within bipolar and unipolar leads are needed to evaluate this.
The paced chamber was not a significant factor in its survival, as there was no difference between pacing characteristics of leads on different chambers, whether at discharge or at follow-up time or regarding bipolar steroid-eluting and unipolar non-steroid-eluting leads, which is consistent with previous studies.1 However, the pacing threshold significantly increased regarding unipolar non-steroid-eluting leads on the left ventricle or right atrium, but not on the right ventricle, probably due to the sample population, since there was an increase in pacing threshold in the right ventricular unipolar non-steroid-eluting leads but not statistically significant.
Our findings indicated that epicardial leads implemented in adults and children were not significantly different in long-term function. Consistent with our results, adults and children with previous CHD were compared in a study that showed no difference in epicardial and endocardial survival regarding age groups.15 Although in that study, bipolarity and steroid elution were not investigated separately, the lead type (bipolar and unipolar) adjusted analysis of age variable in the current study similarly was not correlated with lead survival. In contrast, the longevity of a lead function was significantly better in over 12-year-old children compared with younger ones.14 Therefore, investigating lead characteristics within children might change the results; however, we did the same analysis, which showed no association between age groups among children and better survival.
Congenital heart defect which is an indication for epicardial lead implantation observed in half of our study population but was not associated with the long-term function of a lead.1,2However, a study on patients with endocardial and epicardial leads in pediatrics illustrated that congenital heart defect is a significant predictive of lead failure in both type of the leads, which mostly resulted in failure due to exit block.14
In this study, we compared the survival of bipolar steroid-eluting leads with unipolar non-steroid-eluting ones, which is pivotal for deciding about the type of lead to be used, particularly in patients that endocardial leads are not applicable. Also, the function of epicardial leads is not different between adults and children; thus, they could be used in both groups, despite much of the previous studies that investigated their longevity in children only.
We encountered some limitations due to patients who were not presented for regular follow up, or not at specific time. However, this centre is referral and patients who present from various regions in the country, usually prefer to get their follow up in their living area and refer to this centre in case of pacemaker problems and failures. Therefore, it could be assumed that their pacemaker was functional during the time of their absence. Furthermore, to solve this limitation, we called these patients and completed their follow up by receiving their documents as much as possible.